Nuclear magnetic resonance (NMR) causes spins to excite and then to relax. The relaxation may be T1 (spin-lattice) relaxation or T2 (spin-spin) relaxation. Theoretically, data acquired during the relaxation ought to follow idealized curves. In practice, data that produces less than ideal relaxation curves may be acquired. Conventional T2 mapping in magnetic resonance imaging (MRI) may employ a spin echo with multiple echoes (SEMC) approach like the Carr-Purcell-Meiboom-Gill (CPMG) sequence. These conventional approaches may be negatively impacted by a slice profile effect that artificially lowers the signal of a first echo and by a stimulated echo effect that artificially raises the signal for even echoes.
T2 mapping may be used in MRI to reveal biomarkers that may identify pathological changes in a patient. Conventional T2 mapping may have produced qualitative images that were suitable for subjective interpretation by radiologists. However, conventional T2 mapping may have produced sub-optimal or even unacceptable results when quantitative T2 analysis was desired.
Recall that in MRI, B0 refers to the constant, homogeneous magnetic field used to polarize spins, creating magnetization. This can refer to both the direction and the magnitude of the field. The direction of B0 defines the longitudinal axis. Recall also that B1 refers to a radio frequency (RF) energy field applied perpendicular to the longitudinal axis (B0) to perturb the magnetization in some manner (e.g., excitation pulses, inversion pulses).
Conventional SEMC techniques have faced at least two challenges. First, SEMC may have a non-ideal slice profile for a two dimensional (2D) acquisition. Second, SEMC may undesirably stimulate echoes due to an inhomogeneous B1 field that causes imperfect refocusing of 180 degree refocusing RF pulses. The imperfectly refocused pulses may stimulate undesired echoes. These two challenges may have caused what should have been a purely exponential decay curve for T2 data to be less than the predicted ideal.
In SEMC, slice profiles and the effects of B1 inhomogeneity may vary from MRI system to MRI system. Additionally, in SEMC, the effects of B1 inhomogeneity and non-ideal slice profile may vary based on different imaging protocol parameter selections. Given different imaging parameters on different systems, a T2 map produced using SEMC may have significant deviations from an ideal.
Prior Art FIG. 1 illustrates a traditional spin-echo pulse sequence that produces multiple echoes for T2-mapping. In this conventional approach, T2 weighting and acquisition are mixed together. Mixing T2 weighting and acquisition may have reduced acquisition time, but may also have produced additive errors that compounded for later echoes. An acquisition scheme of four different T2 weighted images (I1, I2, I3, I4) with different echo times (TE) are illustrated. With ideal CPMG-SEMC sequence conditions, signals with different TEs should, theoretically, present mono-exponential decay curves for single uniform protons. T2 parameter maps could then be calculated by fitting to the mono-exponential decay model. Unfortunately conventional SEMC approaches have not yielded the mono-exponential decay curves.
The traditional spin echo pulse sequence in prior art FIG. 1 includes an initial 90 degree RF pulse 110 that is active while the slice select gradient GS is active. Phase encoding may then be applied using a phase gradient GP and read gradient GR. A 180 degree RF pulse 120 may then be applied while the slice select gradient GS is active. A first echo S1 may then be acquired while the readout gradient GR is active. S1 will typically be too low due to the imperfect slice profile effect. Echoes S2, S3, and S4 may then be acquired while the readout gradient GR is active following 180 degree refocusing RF pulses 130, 140, and 150. S2 and S4 will typically be too high due to the stimulated echo effect.
Conventional CPMG-SEMC that uses slice selective RF pulses for 2D acquisitions yield imperfect slice profiles. FIG. 2 illustrates an example imperfect slice profile 220. A 2D slice select pulse 200 may have been intended to produce an ideal slice profile 210 but may instead have produced an imperfect slice profile 220. The effect of the imperfect slice profile 220 may vary inversely with the size of the slice. For example, a thinner slice may experience a greater impact than a thicker slice. The effective flip angle within a desired slice may not be homogenous, particularly at the excitation transition. Additionally, since the B1 field may not be homogenous in a selected volume, and since the B1 field is related to the refocusing RF pulse, the different echo signals that are acquired may produce non-ideal decay curves. For example, when the B1 field is inhomogeneous, in some locations the 180 degree refocusing RF pulse may achieve a flip angle of 180 degrees but in other locations the flip angle may be less than 180 degrees.
FIG. 3 illustrates data associated with an actual T2-related decay that is not an ideal exponential function. The data in FIG. 3 was acquired using conventional CPMG-SEMC with slice excitation and refocusing pulses. In this type of acquisition the first echo 310 may provide lower than ideal signal intensity due to an imperfect slice profile. For example, the excitation and first refocusing RF pulse may cause a first echo 310 to have a lower than expected or lower than ideal signal intensity. Additionally, even echoes (e.g., 320, 340) may produce higher than ideal signal intensities due to superimposed echoes. For example, when the refocusing RF pulse flip angle is less than 180 degrees due to the non-ideal slice profile, stimulated echoes may superimpose on even echo signals and thus yield a higher than expected or higher than ideal signal intensity.
FIG. 4 illustrates a simplified view of errors associated with a conventional pulse sequence. An ideal exponential curve 400 is provided. Initial echo 410 may depend on an initial 90 degree RF pulse (e.g., pulse 110, FIG. 1) and an initial 180 degree RF pulse (e.g., pulse 120, FIG. 1). Initial echo 410 will typically be lower than it should be due to the imperfect slice profile effect. Second echo 420 may depend on an initial 90 degree RF pulse (e.g., pulse 110, FIG. 1), an initial 180 degree RF pulse (e.g., pulse 120, FIG. 1), and a subsequent 180 degree RF pulse (e.g., pulse 130, FIG. 1). Second echo 420 may be larger than it should be due to the stimulated echo effect. Note that the second echo 420 depends on three RF pulses, each of which may introduce some error. Thus, there may be more compound error effects on second echo 420 than on first echo 410. Additionally, the errors may be different for the different echoes, which may make it difficult, if even possible at all, to account for the variable and compounding errors. Third echo 430 may depend on an initial 90 degree RF pulse (e.g., pulse 110, FIG. 1), an initial 180 degree RF pulse (e.g., pulse 120, FIG. 1), a subsequent 180 degree RF pulse (e.g., pulse 130, FIG. 1), and another subsequent 180 degree RF pulse (e.g., pulse 140, FIG. 1). Third echo 430 may be different than it should be due to compounded errors. Note that the third echo 430 depends on four RF pulses, each of which may introduce some different error. Thus, there may be more compound error effects on third echo 430 than on earlier echoes. Fourth echo 440 may depend on an initial 90 degree RF pulse (e.g., pulse 110, FIG. 1), an initial 180 degree RF pulse (e.g., pulse 120, FIG. 1), a subsequent 180 degree RF pulse (e.g., pulse 130, FIG. 1), another subsequent 180 degree RF pulse (e.g., pulse 140, FIG. 1), and another subsequent 180 degree RF pulse (e.g., pulse 150, FIG. 1). Fourth echo 440 may be different than it should be due to compounded errors and may be greater than it should be due to the stimulated echo effect. Note that the fourth echo 440 depends on five RF pulses, each of which may introduce some different error. Thus, there may be more compound error effects on the fourth echo 440 than on earlier echoes. While conventional spin echo sequences that acquired multiple echoes may have reduced acquisition time, the value of the later echoes may have been questionable.
The accuracy of the T2 mapping associated with a conventional sequence may depend most on the first echo 410 and the second echo 420. However, both of these echoes exhibit significant errors. Thus, not only may the later echoes have had questionable value, but the first two echoes, which are most significant to T2 accuracy, may have been fundamentally flawed. Additionally, since the later echoes depend on multiple errors that may have each been different, it may be difficult, if even possible at all, to account for the variable and compounding errors.
The fluctuations and deviations may be related to the T2 value(s) for a given tissue. For example, larger T2 values for the measured species may produce larger deviations. FIG. 5 illustrates the impact of T2 values on system performance. FIG. 5 shows that species having longer T2 and/or T1 values may present more significant deviations than species having shorter T2 and/or T1 values. More generally, the magnitude of the slice select effect and the magnitude of the stimulated echo effect may vary directly with the T2 length.
Conventional attempts to address these issues with 2D CPMG-SEMC have included elaborately designed RF pulses and post-processing. The refinement and elegance of elaborately designed RF pulses may have been limited by bandwidth considerations, digitization considerations, and system performance (e.g., RF chain). Post-processing may have attempted to correct for the stimulated echo effect by modeling signal pathways or selectively using different echo signals for exponential fitting. Post-processing methods may still be vulnerable to MRI system performance, image parameter selections, and T2 ranges. While substantial efforts may have been attempted, the systemic problems and variably compounding errors may have limited the value of conventional approaches for quantitative T2 mapping.